1. Field of the Invention
This invention relates to ultrasonic diagnostic imaging using Doppler shift measurement for detection and display of fluid flow velocities, and more particularly, to an imaging system in which undesirable image artifacts are eliminated by use of an image registration method that enables accurate image motion measurement.
2. Description of Related Art
Ultrasonic imaging techniques are commonly used to produce two-dimensional diagnostic images of internal features of an object, such as a human anatomy. A diagnostic ultrasonic imaging system for medical use forms images of internal tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that travel into the body. The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities or impedance changes to the propagating ultrasonic pulses. These echoes return to the transducer, and are converted back into electrical signals that are amplified and decoded to produce a cross-sectional image of the tissues. These ultrasonic imaging systems are of significant importance to the medical field by providing physicians with real-time, high resolution images of the internal features of a human anatomy without resort to more invasive exploratory techniques, such as surgery.
The acoustic transducer which radiates the ultrasonic pulses typically comprises a piezoelectric element or matrix of piezoelectric elements. As known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the ultrasonic pulses. In a similar manner, the received echoes cause the piezoelectric element to deform and generate the corresponding electrical signal. The acoustic transducer may be packaged within a handheld device that allows the physician substantial freedom to manipulate the transducer easily over a desired area of interest. The transducer would then be electrically connected via a cable to a central control device that generates and processes the electrical signals. In turn, the control device transmits the image information to a real-time viewing device, such as a video display terminal (VDT). The image information may also be stored to enable other physicians to view the diagnostic images at a later date.
One particular application of ultrasonic diagnostic imaging takes advantage of Doppler shift measurement to detect and display fluid flow velocities. In such a system, a region of interest within a patient is repetitively pulsed with ultrasonic signals, and the received echo signals are compared with a reference to determine a rate of flow of fluids through the region. The rate of flow can be determined from a measurement of the Doppler frequency shift of the received echo signals, such as disclosed in U.S. Pat. No. 4,800,891, issued to Kim, for DOPPLER VELOCITY PROCESSING METHOD AND APPARATUS. As known in the art, the flow velocity can then be displayed within a colorized cross-sectional image in which different shading and color intensity represents flow rate and direction. These ultrasonic color flow imaging systems are particularly advantageous in detecting blood flow through a vessel, such as within the heart.
A drawback of Doppler flow measurement systems is that they cannot easily distinguish between fluid flow and other types of movement, such as due to transducer or tissue motion. As a result, any relative motion between the transducer and the region of interest can be improperly interpreted as fluid flow. Such Doppler flow measurement systems are therefore susceptible to an imaging artifact referred to as "color flashing," in which color flow image data is displayed in regions where there is no actual flow. Slight movement of the transducer changes the ultrasonic path length by an amount sufficient to cause a Doppler frequency shift that is similar to the Doppler shift generated by blood flow. The motion induced artifact appears on the image display as a flashing or flickering of the colorized portion of the image. This color flashing can significantly hinder ultrasound diagnosis by yielding false motion indications. While it is possible to mitigate the color flashing by keeping the transducer motion to a minimum, a physician may find it desirable to intentionally move the transducer along the skin surface while simultaneously observing the blood flow rate through a particular vessel.
In conventional color flow imaging systems, removal of signals arising from tissue movement is accomplished with a high pass filter. The filter characteristics incorporate certain assumptions about the maximum frequency of tissue motion signals which determine the necessary cut-off frequency of the filter. In practice, however, the frequency spectrum of the tissue motion signals is not stationary, and cannot be entirely eliminated by the filter without losing some desirable fluid flow motion information. Transducer motion signals tend to be even more complex and less predictable, and thus are considerably more difficult to remove through use of conventional filtering techniques.
Various signal processing techniques have been proposed for eliminating unwanted signals within ultrasonic imaging systems, such as U.S. Pat. No. 4,961,427, issued to Namekawa, for ULTRASONIC DOPPLER DIAGNOSTIC APPARATUS. Namekawa discloses an approach to compensating for unwanted signal clutter during signal down-conversion to baseband using an RF signal mixer. Since the baseband signal is complex (i.e., having in-phase (I) and quadrature (Q) components), tissue motion compensation is accomplished using a tissue velocity signal as the complex reference signal. Unfortunately, this approach provides only limited effectiveness in compensating for tissue motion, since it does not account for spectral changes in the received signal other than mean tissue frequency, e.g., tissue motion signal power and spectral width. Moreover, the prior art techniques do not compensate for signal components that result from transducer motion, whether intentional or unintentional.
Accordingly, a critical need exists for an ultrasonic diagnostic imaging system for detection and display of fluid flow velocities that is capable of compensating for both tissue and transducer motion. The imaging system should permit a physician to readily manipulate the transducer across a region of interest while simultaneously monitoring fluid flow rates and direction.